Fluoride borosilicate glass having low refraction an optical path independent of temperature and an elevated dispersion

ABSTRACT

FLUORIDE BOROSILICATE GLASS CHARCTERIZED BY A LOW INDEX OF REFRACTION, AN OPTICAL PATH INDEPENDANT OF TEMPERATURE AND AN ELEVATED DISPERSION CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT:   SIO2 14-63 B2O3 6-20 AL2O3 5-18 ALKALI OXIDE PLUS ALKALI FLUORIDE 8-20 SB2O3 1.5-32 PLUS ADDITIONAL FLUORINE, AS FLUORIDES, CALCULATED AS ELEMENTAL FLUORINE 1-12

United States Patent US. Cl. 106-54 19 Claims ABSTRACT OF THE DISCLOSURE Fluoride borosilicate glass characterized by a low index of refraction, an optical path independent of temperature and an elevated dispersion consisting essentially of, in weight percent:

SiO 14-63 2 3 6-20 A1 0 -18 Alkali oxide plus alkali fluoride 8-20 Sb O 1.5-32 Plus additional fluorine, as fluorides, calculated as elemental fluorine l-l2 This application is a continuation of Ser. No. 820,007, filed Apr. 28, 1969, now abandoned.

BACKGROUND This invention relates to fluoride borosilicate glass having a low refractive index, an optical path substantially independent of temperature and an elevated dispersion.

It is known that wavefronts are deformed in glasses when the optical path varies as a result of temperature gradients occurring at different points in the glass.

Optical instruments forming part of the equipment of space vehicles, for example, are exposed to great temperature differences by alternate solar radiation followed by loss of radiant heat in space, and also by emitted and/ or reflected infrared radiation from close planets. These temperature difference produce wa ve deformations in optical glasses, thereby diminishing the resolving power of the optical systems.

Even in aerial photography, the temperature gradients that occur at high altitudes can considerably detract from resolving power.

There is, therefore, a great need to provide optical systems and optical windows from glass compositions having optical paths that are independent of temperature.

In planoparallel aircraft and spacecraft windows, for example, the optical path variation resulting from the temperature variation is expressed by the following formula:

An undesirable wavefront deformation is produced when AW varies in magnitude as a result of temperature gradients occurring at different points in the window. The above formula shows that the optical path difference AW can be reduced by reducing the thickness d of the glass element, the temperature difference At, or the magnitude of dn Ga(nl)+ The magnitude of G depends only on the physical prop- Patented Oct. 9, 1973 erties of the glass and is required to be as small as possible and ideally should be 0. The value of dn/dt must therefore be negative. This would be made possible by the introduction of glass components:

(1) Which produce an optimal elevation of the coeflicient of thermal expansion,

(2a) Which affect the glass in such a manner that the temperature-dependence of the natural ultraviolet frequency (dAo/dt) remains as low as possible, and/or (2b) Which bring about a shift of the natural ultra violet frequencies A of the glass towards the short wavelengths.

However, the increase in the thermal expansion, which is necessary according to 1), makes it difiicult to meet the requirement that G be equal to zero, as, due to the resultant increase in the term a (n-l), the temperature coefficient dn/dt has to assume very high negative values. This conflict clearly shows the diificulties affecting the development of glasses having an optical path that is independent of temperature THE INVENTION It is a primary object of the present invention to provide glass compositions in which the disturbing effect of a temperature gradient on the image-making properties is prevented.

It is another object of the present invention to provide glasses having low refractive indices, elevated dispersions and optical paths substantially independent of temperature.

These and other objects of the invention are realized by glasses having the following composition in percent by weight and the following properties:

and F in amount of 0-5.6 wt. percent, batch basis, of the balance of the batch, said glass having a negative dnI/dt value, an index of refraction n of 1.4619-1.5334, and an Abbe valve v of 44.7-68.4.

Fluoride can be added to the melt in the form of a fluoride, for example, as KF, A11 a bifiuoride, as for instance KHF or in another form, as for example, a silico fluoride.

In glass compositions which have been prepared on a basis of SiO;,, the components fluorine, B 0 and K 0 produce a negative shift in the dn/dt values without substantially increasing the thermal expansion.

The fluorine component also has the effect that the temperature independence of the ultraviolet natural frequency dAo/dt is minimized. An increasing B 0 content shifts the ultraviolet natural frequency towards the short wave lengths, and Sb O does not have any unfavorable influence on the ultraviolet natural frequency A according to the invention and their physical properties 0' The following examples of compositions of the glasses v re given forthe. purpose of illustrating the present in- TABLE is claimed isi 1. Fluoride jborosili ate glasscharacterized an optical path independent of ternperature and an elevated dis- Exarnple 1 2 3 4 5 6 7 8 S10, 48. 9 49. 8 44. 5 41. 5 37. 3 38. 1 v 36. 5 37. 14. 9 17. 2 17.8 15. 2 15. 3 17. 7 13. 2 9. 6 G. 7. 1 14. 3 15. 2 13. 7 14. 4 12. 8 V 11. 5

""" "iii""""'6f""""if4 0.3 0.3 0.5 0.1 2.0 6.2 6.3 3.5 3.3 3.0 3.1 2.9 12.5 9. 8 4. 7 4. 7 4. 7 14. 6 9. 5 19. 7 13. 8 12. 5 13. 4 12. 6 16. 6 13. 7 14. 6 12. 9 0.8 0 9 0.9 2.9 1.8 1.8 1.6 13.3

Nbz05 a- 2( O4)a---- PbO 95 90 108 100 3. 3 4. 3 -4. 8 -4. 5 Gabs.=[(n1)a+An +1. 3 0. 1 +0. 2 +0. 4

Example 9 10 11 12 13 14 15 16 17 18 l Fin addition to that in KB and AIR, as fluoride; calculated as F; wt. percent based on balance of batch. 1 Change in refraction value measured in vacuum at 2H0 C. and ).=54fi.1 nm.

The glass compositions of the invention can be prepared by the conventional methods well known to the artisan. An illustrative example of a procedure for preparing glasses according to the invention follows:

A batch of 750 kg. of silica, 460 kg. of boron trioxide, 145 kg. of aluminum hydroxide, 8 kg. of sodium carbonate, 140 kg. of potassium carbonate, 71 kg. of antimony trioxide, 201 kg. of potassium fluoride, 13.5 'kg. of aluminum fluoride and 4.5 kg. of arsenic trioxide was mixed thoroughly in a standard commercial mixer. Following the mixing, the batch was placed in a ceramic pot furnace and completely fused at 1380 C. This took aboutlZ hours The melt was then refined for 8 hours at 1400 C. Thereafter, the furnace temperature was reduced with constant stirring, until the glass had a viscosity of about 400 poises and was suitable for casting. The glass was then poured into a preheated iron mold and cooleddovvn to room-temperature. A glass was obtained which corresponds to Example 1 in the table. The cooling from the transformation point Tg of 360 C. down to room temperature was carried out at 1 C. per hour, or over a total period of 14 days.

persion consisting essentially of, in wt. percent, batch basis:

and F in amount of 0-5.6 wt. percent, batch basis, of the balance of the batch, said glass having a negative dn/dt value, an index'of refraction 2r of 14619-15334, and an Abbe value v of 44.7-68.4.

2. Flouride borosilicate glass according to claim I having the following composition:

Wt. percent Si 48.9 B 0 14.9- A1 0 6.5 Na O 0.1 K 0 6.2 Sb O 9.8 KF 12.5 AlF 0.8 AS203 0-3 and F in amount of 4.3 wt. percent, batch basis, of the balance of the batch.

3. Fluoride borosilicate glass according to claim 1 having the following composition:

Wt. percent SiO 49.8

B 0 17.2 A1 0 7.1 Na O 0.3 K 0 6.3 Sb O 4.7 vKF 13.4 AlF 0.9 AS203 0.3

and F in amount of 4.5 wt. percent, batch basis, of the balance of the batch.

4. Fluoride borosilicate glass according to claim 1 having the following composition:

Wt. percent SiO 44.5 B 0 17.8

A1 0 p V 14.3 Na O 1.4 K 0 3.5 251-30 4.;

AlF 0.9 AS203 0.3

and F in amount of 4.2 wt. percent, batch basis, of the balance of the batch.

5. Fluoride borosilicate glass according to claim I having the following composition:

Wt. percent Si0 41.5 B 0 15.2 A1203" Na O 0.3 K 0 3.3 Sb O 4.7 KF 16.6

AlF 2.9 As O 0.3

and F in amount of 5.6 wt. percent, batch basis, of the balance of the batch.

6. Fluoride borosilicate glass according to claim I having the following composition: I

Wt. percent SiO 37.3 B 0 15.3 A1 0 13.7 N320 0.3 K 0 3.0 Sb O KF A11 1.8 As O 0.3

and F in amount of 4.5 wt. percent, batch basis, of the balance of the batch.

7. Fluoride borosilicate glass according to claim 1 having the following composition:

Wt. percent SiO 38 1 E 0 17.7 A1 0 14.4

N320 K 0 3.1 2 3 132 KP A11 u 1.8 AS203 0.3

and F in amount of 4.8 wt. percent, batch basis, of the balance of the batch.

8. Fluoride borosilicate glass according to claim 1 having the following composition:

and F in amount of 4.3 percent, batch basis, of the balance of the batch.

9. Fluoride borosilicate glass according to claim I having the following composition:

Wt. percent Si0 37.0 B 0 9.6 A1 0 11.5 Na O 2.0 K 0 12.5 5 3 0 13.8 AlF 13.3 AS203 0.3

and F. in amount of 3.8 Wt. percent, batch basis, of the balance of the batch.

10. Fluoride borosilicate glass according to claim I having the following composition:

Wt. percent Si0 36.2 B 0 7.2 A1 0 112 Na O 125 Sb O AlF 18.3 PbO 4.3 AS203 11. Fluoride borosilicate glass according to claim I having the following composition:

Wt. percent SiO 34.0 B 0 8.7 A1 0 9.2 K 0 11.0 IS IDP%O3' 22.2 3.1 AlF 11.5 AS203 and F in amount of 1.2 wt. percent, batch basis, of the balance of the batch.

12. Fluoride borosilicate glass according to claim I having the following composition:

Wt. percent SiO 34.8 B 16.9 A1 0 9.8 N320 3.5 K 0 10.9 813 0 9.5 AiF, 14.3 AS203 O-3 13. Fluoride borosilicate glass according to claim I having the following composition:

Wt. percent SiO 29.2 B 0 11.0 A1 0 10.8 Na O 0.1 K 0 1.2 $21 i3;

AlF 1.4 Al (SO 15.6 AS203 0.3

and F in amount of 4.8 wt. percent, batch basis, of the balance of the batch.

14. Fluoride borosilicate glass according to claim I having the following composition:

Wt. percent Si0 38.3 B 0 11.0 A1 0 12.3 N320 0.1 K 0 8.7 @83 3 2.;

ME. 1.9 W0 0.1 PbO 0.5 AS203 0.3

and F in amount of 2.5 wt. percent, batch basis, of the balance of the batch.

and F in amount of 2.5 wt. percent, batch basis, of the balance of the batch.

16. Fluoride borosilicate glass according to claim 1 having the following composition: Wt. percent S 34.6 B 0 10.3 A1 0 11.4 N320 0.]. K 0 8.1 817 0 25.1

Wt. percent KF 6.6 AlFg 1.5 W0 0.5 Nbg05 1.1 PM) 0.4 AS203 0. 3

and F in amount of 2.5 wt. percent, batch basis, of the balance of the batch.

17. Fluoride borosilicate glass according to claim 1 having the following composition:

Wt. percent SiO 25. 1 B 0 17.4 A1 0 14.3 Li O 0.3 Na O 0.5 K 0 1.5 15$ 12'? AIR, 6.0 AS203 0.3

and F in amount of 5.5 wt. percent, batch basis, of the balance of the batch.

18. Fluoride borosilicate glass according to claim 1 having the following composition:

Wt. percent Si0 15 .8 B 0 15.1 A1 0 14.3 Li 0 0.3 N320 0.5 K 0 1 .5 1% i2? s 6.0 As 0 3.0

and F in amount of 5.5 wt. percent, batch basis, of the balance of the batch.

19. Fluoride borosilicate glass according to claim 1 having the following composition:

Wt. percent SiO 5 7.6 B 0 14.0 A1 0 5.3 N320 0. 1 K 0 6.5 ffii 113 AlF 0.3 C8 0 1.0 AS 0 0.2 R13 0 1.0

and F in amount of 4.3 wt. percent, batch basis, of the balance of the batch.

References Cited UNITED STATES PATENTS 3,020,165 2/ 1962 Davis 106-54 3,151,982 10/1964 Corrsin 106- 54 WINSTON A. DOULAS, Primary Examiner M. L. BELL, Assistant Examiner US. Cl. X.R. 10653 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. r a d October 9, 197;

' Inventor) W111i Ritze and Franz Reitmayer It is certified that error appears in the above-identified patent 1 and that said Letters Patent are herebycorrected as shown below:

: Q (301:3 and 4 the Table, Example 2, for

- A1F "'0 9" should be -0.9".

2. C01. 3, line 72, change "Example 1" to --Examp1e2- I v 3. C01 4, line 2, after "characterized' insert "by- Signed end sealed this 14th day of January- 1975.

(SEAL) Attest: v McCOY GIBSON JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PC4050 (10-69) Q I ugcoymppc 50319 .l. G OIIIIIIIY "II'II. O'IICI I... '..l 

